1. Field of the Invention
This invention relates to a ceramic heater which is utilized as a heater by itself or as a heater unit for heating an oxygen sensor by arranging a resistance heating element on an end portion of an elongated plate-like ceramic substrate, and more particularly to an improvement of a lead portion for supplying power to the ceramic heater.
2. Related Art Statement
Heretofore, ceramic heaters obtained by arranging a resistance heating element on an end portion of an elongated rod-like or plate-like ceramic substrate have weidely been used as an insertion type heater or a heater unit for heating an oxygen sensor.
Particularly, there are oxygen sensors used for detecting an oxygen concentration in an exhaust gas of an automotive vehicle, wherein the ceramic heater obtained by arranging the resistance heating element on the end portion of the elongated plate-like ceramic substrate is united with a detecting element so as to heat a detecting portion.
Such an oxygen sensor has a structure as shown, for example, in FIGS. 5 and 6.
In FIG. 5 is shown an exploded view of an oxygen sensor element 40. This element 40 generally comprises a sensor unit 50 and a heater unit 60.
In the sensor unit 50, an outer shell is formed by stacking an elongated plate-like solid electrolyte body 51, which is made from an oxygen ion-conducting solid electrolyte consisting mainly of zirconia, on a solid electrolyte plate 56 made from the same material and provided with a groove 57 extending longitudinally from one end of the plate up to the neighborhood of the other end thereof.
To one end of the front surface of the solid electrolyte body 51 is applied to porous measuring electrode 52 made from platinum or the like by printing, while a reference electrode 55 made from the same material is applied to the rear surface of the solid electrolyte body 51 so as to be oppositely located from the measuring electrode 52.
Two lead members 58, 59 each made from the same material as the electrodes 52, 55 are applied in the form of a strip to both surfaces of the solid electrolyte body 51 so a to longitudinally extend from the electrodes 52, 55 to the other end of the body 51, whereby the end portions of these lead members are a reference electrode terminal 53 and a measuring electrode terminal 54, respectively.
In the heater unit 60, two elongated plate-like insulative ceramic bodies 61, 62 are stacked one upon the other, and a resistance heating element 65 is interposed therebetween at their one end portion. Further, two strip-like conductive lead members 66, 67 are interposed between the insulative ceramic bodies 61 and 62 so as to longitudinally extend from both poles of the resistance heating element 65 to the other end of the insulative ceramic bodies 61, 62.
Since the length of the insulative ceramic body 62 is shorter than that of the insulative ceramic body 61, the end portions of the lead members 66 and 67 are exposed from the insulative ceramic body 62 to form connection terminals 63, 64 for connecting to an external power source.
The heater unit 60 is secured to one side of the sensor unit 50 to form an oxygen sensor element 50 provided with a heater.
As shown in FIG. 6, the middle portion of the oxygen sensor element 40 is placed into a longitudinal square hole (not shown) formed in a center of a porcelain insulator 72a and the neighborhood of the detecting portion thereof, thereby exposing it to a gas to be detected such as an exhaust gas or the like, is put into a longitudinal square hole (not shown) formed in a center of a porcelain insulator 72b, which is housed in a hollow protective tube 71 made of a metal. The oxygen sensor element 40 is settled by a filler 73 such as cement, talc, glass or the like and a filler 78 such as talc or the like, which are filled in the upper and lower side of the porcelain insulator 72a.
In the front end portion 71a of the protective tube 71 are formed a plurality of openings 75, whereby the measuring electrode 52, which is located at the end portion of the oxygen sensor element 40 at the front end portion 71a, is contacted with the exhaust gas. Into a back end portion 71b of the protective tube 71 is placed a rubber stopper 76. Lead wires 77a-77c passing through the rubber stopper 76 and a ground wire 77d connected to the protective tube 71 are connected to respective spring pawl members 79a-79d disposed in a porcelain connector 74. Each of these pawl members is electrically contacted with each of the conenction terminals 63, 64 of the heater unit 60, the reference electrode terminal 53 and the measuring electrode terminal 54 provided on the oxygen sensor element 40.
For instance, the oxygen sensor 70 of the above structure is used to detect an oxygen concentration in an exhaust gas by inserting the front end portion 71a of the protective tube 71 into an exhaust pipe of an automotive vehicle. In this case, the resistance heating element 65 in the heater unit 60 is energized to heat the region of the sensor unit 50 including the electrodes 52 and 55 so as to activate the sensor unit 50, whereby the detection accuracy and response of the sensor can be enhanced particularly when the temperature of the exhaust gas is as low as not more than 500.degree. C.
However, when the exhuast gas temperature is as high as about 900.degree. C., the temperature of the front end portion 71a becomes higher and hence the temperature of the connector portion is fairly high due to thermal conduction along the length of the unit and consequently there may be degradation of the terminal wires and the like. For this reason, the use of the elongated type oxygen sensor 70, as described above, becomes advantageous from a viewpoint that the electrically conductive reliability is enhanced by the protection of the lead wires 77a-77d and spring pawl members 79a-79d.
In such an elongated type oxygen sensor, since the length of the lead members 66, 67 becomes long, the resistance of each of these lead members 66, 67 becomes large and consequently the amount of power consumed by the lead members 66, 67 in power supplied to the heater unit including the lead members increases. In order to maintain the detecting portion of the oxygen sensor element above a given temperature, therefore, large power is required as compared with the case of using the sensor element provided with a short lead member.
When the lead members 66, 67 are made from the same material as the resistance heating element 65 in the above oxygen sensor 70, there is adopted a method wherein the width D.sub.1 of each of the lead members 66, 67 is widened to reduce the resistance of the lead member 66, 67 in order to decrease the power consumption of these lead members.
However, if it is intended to widen the width of the lead member 66, 67, when firing the assembled oxygen sensor element 40, the problems, as mentioned below, result. That is, when the resistance heating element 65 and the lead members 66, 67 are interposed between the two unfired insulative ceramic layers 61 and 62 and then fired at a state of adhering to the unfired sensor unit 50, since a ratio of total area of the lead members 66, 67 to area of the upper and lower insulative ceramic layers 61, 62 is large, the direct contact or joining area between the upper and lower insulative ceramic layers 61 and 62 becomes small and consequently the joint strength after the firing is lowered (i.e. the bonding force in the region containing the lead members 66, 67 is weak after the firing). Further, since there is usually a difference in the firing shrinkage between the lead member 66, 67 and the insulative ceramic layer 61, 62, the ceramic layers become warped after the firing, so that some cracking between the lead member and the ceramic layer or the peeling of the lead member or the creamic layer may result.
Moreover, since a noble metal such as platinum or the like is used as a component for the lead members 66 and 67, when the width of the lead member 66, 67 is widened, the amount of platinum used becomes large, resulting in the increase of the cost.
The aforementioned problems also occur even when using the heater unit having a long lead member as a heater itself in addition to the above elongated type oxygen sensor.